The tips of blades of a turbine need to be shrouded for a number of reasons. These include:
(a) preventing steam from leaking out the blade tips, PA1 (b) acting as a sealing surface, PA1 (c) permitting tuning of blade frequencies, PA1 (d) reducing the bending moment at the blade base and fastening, PA1 (e) increasing mechanical damping of blade vibrations, and PA1 (f) attenuating totally some resonant modes of vibration.
This invention relates to providing a long arc shroud to tenons located at the tips of the blades such that the pitch of the blades can be accurately related to the shroud.
The extremely accurate measurement of the relative position of blade tenons on steam turbine blades has not been necessary when the shroud or cover groupings are small, namely 12 or less holes per segment, because errors made during the measurement process are cancelled at the end of each segment. Thus, small errors in measurement have persisted and are generally tolerable. With long arc shrouding, measurement errors must be avoided, since they can be cumulative and excessive. This can cause unacceptable distortion of the blades, increasing in magnitude as the distance from the center of the arc is increased.
Properly sized long arc covers can be considerably longer than the typically used short arc covers. For example, on four different rows of blades, the number of holes in each segment changed from 4 to 25, 5 to 43, 6 to 26, 7 to 73 and 6 to 91. The purpose of this substantial change in arc length is to suppress the tangential in-phase mode of vibration, which has most frequently been the cause of blade fatigue failures. The benefits of long arcs, harmonic arcs, and continuous harmonic arcs have been described previously.
One of the most accurate ways to locate the tenon holes in a shroud or cover is to place the segment directly on the blade tips next to the projecting tenons. The tenon pitches can then be transferred directly to the part. This technique has been used for many years where cylindrical blade tips have been used. The band is rolled to follow the curvature of the blade tips and is tightly held in place to either the leading or trailing tip surfaces while the pitches are transferred to the blued outer surface of the band using a machinist's square. Axial misalignment and rotational variations in the tenons can also be transferred at this time.
With conical blade tips, a degree of error occurs when pitches are transferred in a similar manner. This can generally be tolerated for short segment designs. The shroud, band or cover is a segment of a conical surface, machined from flat stock and then rolled to the desired tip curvature. Pitch error is caused by transferring the pitch at the top of the tenons instead of the bottom of the tenon.
Conical tip blades can have any of three types of tenon attached bands. The outer surface of the band can be parallel with the inner surface of the band which is typical of most GE (Trademark) designs. Alternatively the outer surface of the band has a shallower angle than the inner surface of the band as in some Westinghouse (Trademark) designs. Yet further the outer surface of the band can be cylindrical and the inner surface of the band conical, as is used on many Westinghouse Marine Division designed rows.
Conical bands in which the inner and outer surfaces are parallel are usually made by one of two methods. Short arc segments are usually cut from plate in a crescent-shaped conic section and rolled to the proper curvature after the holes have been made. Long arc segments are usually made by edge rolling strip stock. This causes the material to curve crescent shaped. It is then rolled to the proper curvature, whereupon it becomes a conic segment. With either of these configurations, accurate measurement or transfer of pitches is difficult.
A pitch measurement taken for example, at the top of the tenons introduces an error of 5 mils/blade. For a six-blade group, this amounts to a 0.030", which distributed equally produces a maximum distortion of 0.015" at the end blades of the group. On a 73-blade (120 degree arc segment) group, this error amounts to 0.365", or 0.183" at the end blades. This amount of distortion is unacceptable and would most likely cause premature failure of the tenons. Similar error occurs if the pitches are taken by placing the shroud on the admission or discharge conical surfaces adjacent to the tenons.
Changes occur in the shroud pitches due to punching, riveting, and shroud machining. Changes which occur after the pitches are transferred produce an error, which if excessive or left uncompensated, can cause excessive distortion stress.
An objective of the invention is to provide for accurately locating tenon holes particularly in long arc conical bands.